The present saline loading countermeasures used by astronauts to maintain their orthostatic intolerance are not fully effective. It is hypothesized that vasopressin analogs will improve this. The experimental design will involve overnight head-down bedrest by the subject to reproduce what is thought to occur in astronauts. Blood volumes, orthostatic responses, and fluid balance will be measured in subjects serving as their own controls after random assignment to three treatment periods (LVP, DDAVP, and placebo) 2 wks apart.